I. BALB/c mice are highly susceptible to plasmacytoma (PCT) induction protocols using the myc-raf containing J3V1 retrovirus while DBA/2 mice are resistant. We have previously demonstrated that susceptibility is associated with the BALB/c B cell genotype. However, tumor induction still requires granuloma formation initiated by the prior administration of pristane indicating the necessity for an appropriate microenvironment for neoplastic progression. To examine the role of T cells in this microenvironment, J3V1 was used to induce tumors in nude mice. B lineage tumors in nude mice were almost exclusively pre-B or B cell by phenotype with PCT's occurring only rarely. In contrast, when tumors were induced in T cell reconstituted nude mice, 40% were PCT's and 60% were B cell tumors indistinguishable from those induced in unreconstituted nude mice. These results are interpreted to indicate that J3V1 can transform early B lineage cells and that T cells play no role in the transformation process. T cells are, however, required to drive transformed B cells into fully differentiated, antibody secreting plasma cells. II. The early development of PCT's is currently being addressed by in situ hybridization studies to assess clonality of developing neoplastic lesions. Probes have been prepared to 7 VH families and have been hybridized to serial sections from granulomatous tissue containing putative early stage lesions. Similar studies will additionally enumerate other cell types associated with developing lesions to define the cellular microenvironment associated with this neoplastic process. III. Previous studies have demonstrated that primary plasmacytomas are dependent on physical contact with stromal cell feeder layers for survival and proliferation. This interaction is mediated, in part, by the cell surface molecule CD44, but available antibodies to other mouse cell surface receptors fail to inhibit adhesion. To identify additional molecules involved in this adhesion, rat monoclonal antibodies have been prepared against stromal cells obtained from the primary site of plasmacytomagenesis. These antibodies are currently being screened for specificity and ability to inhibit adhesion of plasma cells to stromal cell feeder layers. The role of CD44 in both neoplastic and normal development is a question of major biological interest. To examine the in vivo function of CD44 we are in the process of generating CD44 knockout mice. A series of genomic clones have been isolated and are currently being analyzed for exon locations. A selected exon will then be interrupted with the neomycin resistance gene and the inactivated allele introduced into embryonal stem cells for generation of chimeric mice.